Materials used in the fabrication of component parts in the aerospace industry must have certain characteristics to protect the parts from damage or hazards caused by common environmental occurrences. Lightning, an example of a common environmental occurrence, can severely damage and/or punch through component parts if such parts are not adequately conductive and grounded through the aircraft. If lightning strikes a wing component of an aircraft during flight, the event has the potential of causing a dangerous surge current in addition to causing serious physical damage of the component itself. The surge current is particularly concerning because it may eventually come into contact with a fuel reservoir causing an explosion to occur. As a result of an actual fatal plane crash caused by a lightning strike, the Federal Aviation Administration (FAA) implemented a system to categorize various zones for commercial aircraft based on probability and severity of being struck by lightning. Thus, it is crucial that such component parts are manufactured to have characteristics which, among other characteristics, prevent or alleviate damage caused by lightning strikes.
Electromagnetic interference (EMI) is another electrical concern of composite parts in the aerospace industry. EMI waves consist of electric and magnetic fields which can induce electrical transients to induce excessive energy levels in the electrical wiring and probes of the fuel system. A method to prevent and/or reduce these occurrences is to add shielding materials to absorb or reflect the impinging radiation. Without proper shielding from these events, the waves can interfere with an aircraft's electronic and avionic equipment operation or even lead to ignition of fuel tanks. Absorption losses have been shown to be proportional to the thickness, conductivity and permeability of the shield material. Conventional shielding methods include housings made from cast and sheet metal, and plastics with conductive fillers or coatings.
Electrostatic discharge (ESD) is yet another concern for composite parts in the aerospace industry. ESD is the sudden and momentary electric current that flows between two objects at different electrical potentials caused by direct contact or induced by an electrostatic field. Non-conductive materials, paints, plastics have insulating properties and therefore are subject to accumulation of static charges. The resulting charges must be controlled to protect aircraft electronics and fuel tanks. Conventional ESD methods include adding fibers which have static elimination characteristics to a material, e.g., carbon fiber, or adding wicks and/or rods at the tips of aircraft components.
Static charge is imparted to a material through friction. An airplane becomes charged simply by passing through the air. Flight through precipitation (clouds or rain) increases charge accumulation, as there is more material contact. Static charge is routinely discharged in air at sea level, which is slightly conductive, and also in air with higher humidity. However, air with humidity below 20 percent and/or at higher altitudes is a poor conductor. The latter permits static charge to build up on aircraft surfaces, especially those of composite aircraft, where charge does not readily move. The build-up of charge on a structure creates a voltage potential that increases with the amount of charge. On metal structures, this voltage potential is the same everywhere because metal conducts electricity evenly. On composite structures, however, the voltage will vary. This voltage potential, in turn, generates an electric field which is most intense at areas of acute curvature such as wing tips, propeller tips, trailing edges, tips and edges of jet engine blades, etc. Built-up charge wants to travel-like charges repel and unlike charges attract. Eventually, the difference in charge between the air and structure becomes so great that the need to discharge the voltage potential takes over, resulting in a mass “dumping” of the excess charge into the atmosphere. Static charge build-up can trigger lightning within clouds or in charged atmospheric conditions.
At the same time, such component parts must be manufactured to target certain weight requirements in order for the aircraft to achieve designed distance and also to overcome the gravitational force of its own weight to gain flight without using an inordinate amount of fuel. Additionally, such component parts must be manufactured to resist damage to common environmental occurrences. This characteristic is generally described as “toughness” with respect to composites. Thus, concerns of damage tolerance and resistance to common environmental occurrences while maintaining a practicable weight of these component parts must be evaluated very carefully in the manufacturing process of such parts. Various methods are used to strike this balance in the manufacturing process.
A conventional method for imparting lightning strike protection to component parts in the aerospace industry is the use of expanded aluminum, copper, titanium or bronze mesh, screen or foils, or woven wire fabrics, incorporated into the composite part. Although such meshes are generally effective as lightning strike protection, many of these expanded mesh/screens are difficult to handle for both production and repairs. Additionally, they generally require isolation materials (e.g., a fiberglass isolator ply) to prevent undesirable galvanic corrosion in the presence of other materials, especially aluminum with carbon composite structures. Moreover, when used in large quantities, expanded mesh/screens are very heavy and may significantly add to the weight of the overall part thereby decreasing the efficiency of the aircraft.
Another method of imparting lightning strike protection is the use of metal-coated carbon fiber material incorporated into the composite part. Generally, the carbon fibers are coated with nickel, palladium, tin, copper or a combination thereof using an electroless plating process. These metal-coated fibers may then be formed into a uniform nonwoven material. The nonwoven material with metal-coated fibers is incorporated into the composite effectively replacing the metal mesh/screen which would otherwise be needed for adequate lightning strike protection. Reports of composite parts having such nonwoven materials with metal-coated fibers therein are reported to have a metal content of between about sixty (60) and one-hundred (100) grams per square meter (gsm) of metal. Other reports cite a 10% to 65% metal by weight content for carbon tows between 6K (6000 filaments) to 80K (80,000 filaments). The metal-coated veil materials are made with metal-coated fibers lightly bonded together with non-conductive resin (e.g., PVA). Thus, the weight of the overall composite still presents issues with respect to efficiency of the aircraft. Moreover, the electroless plating process presents manufacturing issues such as plating waste streams and higher manufacturing costs.